Multiple rotor helicopter control



y 6, 1952 J. 5. NEALE 2,595,875

MULTIPLE ROTOR HELICOPTER CONTROL Filed March 11, 1949' 2 S!-IEETS-SHEET 1 May 6, 1952 J. s. NEALE MULTIPLE ROTOR HELICOPTER CONTROL 2 SHEETSSHEET 2 Filed March 11, 1949 awk kg, M a R Q Patented May 6, 1952 MULTIPLE ROTOR HELICOPTER CONTROL John Sidney Neale, Hendon, London, England,

assignor to Pennine Aircraft Limited, Manchester, England, a British company Application March 11, 1949, Serial No. 80,977 In Great Britain January 4, 1949 4 Claims. 1

This invention relates to multiple rotor helicopter control, and has for its object to provide a convenient arrangement in multi-rotor helicopters for assisting in the equalisation of airflow velocities through the rotor discs, and for providing a basis for increased translational speeds.

In order to obtain maximum efficiency it is desirable that the velocity of airflow through the rotor disc should be uniform over its area, but this is diflicult to achieve with rotors having conven-. tional pitch control systems, particularly in translational flight. In this phase the advancing blade had added to its rotational component the forward speed of the airflow and accordingly the lift with a normal blade has a positive value increasing gradually towards the tip. In the case of the retreating blade, which has a much higher angle of attack, the inner portion experiences reverse flow and may not be lifting, whereas the outer portion shows a steep rise in lift with a high maximum near the tip.

In the arrangement in accordance with this invention, a multi-rotor system is used in which the retreating blades (in normal translation) overlap, the line joining the centres of the rotor discs being substantially at right angles to the normal line of flight, and means are provided to produce cyclic pitch change such as to avoid substantial increase in airflow through the double swept area.

Preferably the blades of each rotor are interconnected by a pitch control system for obtaining torsionally balanced cyclic pitch changes, and in each blade the lever arm between the centre of pressure and the axis for pitch change is arranged to increase towards the blade tip, whereby the relative lift of the retreating blades is reduced and substantial increase in airflow through the double swept area is avoided.

The arrangement has the advantage that while normally the retreating blade at high translational speed is nearly stalled, by reducing the angle of attack it is possible to increase the load on the blade and/or the translational speed for a given load by giving a wider aerofoil working range. Further the bending moment on the blade is reduced, as also is the drag.

Referring to the accompanying explanatory drawing:

Figure l is a diagrammatic plan view of a system of two rotors arranged in accordance with the invention.

Figure 2 shows diagrammatically an arrangement for driving and controlling the rotors.

Figure 3 is a plan of a single rotor blade.

As shown in Figure 1, two rotor systems having blades a, b, c and d, e, f, are arranged to overlap to the extent that the circumference of each rotor disc passes over or near the centre of the other rotor disc. The rotors rotate in the direction of the arrows, and the direction of flight is shown by the arrow 0, so that the overlapping blades are the retreating blades. The line of centres of the rotor disc is at right angles to the line of flight.

The high lift outer portions of retreating blades sweep out complementary areas, and normally the total air inflow over such areas would be double that of the remaining areas. avoid this and to make the inflow as nearly as possible uniform over the whole area, according to the invention use is made of a pitch changing system to reduce the lift of the retreating blades, so that reduced lift is obtained from the blade I, while the blade 0 is reducing its lift and the blade b is increasing its lift, relative to the lift corresponding to the pitch required to equalise the lift at all the blade roots as normally utilised.

As shown in Figure 2, the two rotors are driven from an engine shaft h, through a primary worm gear 1', shafts k with universal couplings m, secondary gear boxes n and rotor shafts 0. Each rotor has a universally mounted head with hydraulic means p for tilting the rotor, having lines to the cockpit for the supply of hydraulic fluid. Each rotor blade'is mounted. in the rotor head to rotate about an axis rs (Figure 3) for pitch change, and each is coupled to a piston q moving in a hydraulic cylinder t. The cylinders t of each rotor are connected by tubes u to a common point in the rotor head,'forming a constant volume hydraulic system, so that when the lift of a blade increases, the greater torque tends to rotate the blade to reduce its pitch and fluid is driven to the cylinders t of the other blades of the same-rotor to rotate them so as to increase their pitch. There is also a line *0 through which hydraulic fluid may be supplied to or withdrawn from each rotor head for general pitch control. Pitch control systems of this type have been described in copending applications by the same applicant viz., Application No. 664,509 filed April 24, 1946 and now abandoned, and Application No. 19,473 filed April 7, 1948 which issued as Patent No. 2,593,335.

As shown in Figure 3, the axis for pitch change rs of each blade is placed at an angle to the centre of pressure axis w--y. the two axes d1- verging towards the tip of the blade. This has In order to 3 the result that with a given lift, the torque acting on the blade is greater near the tip than near the root owing to the increased lever arm between the two axes. Therefore the increased lift at the outer part of the retreating blade is enabled to produce a greater effect in tending to reduce the pitch and hence also the lift of the retreating blade, and the lift produced over the over-lapping parts of the rotor discs is relatively reduced, while the lift over the remaining parts is relatively increased by the action of the hydraulic pitch control system. Consequently the velocity of airflow tends to be decreased in the double swept area and increased in the single swept areas, so that its value becomes more nearly equal over the whole area.

While the system described above for obtaining cyclic pitch changes is the preferred arrangement, it is also possible to make use of a swash plate system in which the rotor blades are turned about the axis for pitch change by levers actuated by a tiltable bearing member positively controlled from the cockpit. In this case the centre ofpressure is preferably not ofiset from the axis for pitch change.

What I claim is:

' 1'. A helicopter having at least one pair of rotorsarranged at a distance apart which is less than the rotor diameter with the line joining the centres of the rotor discs substantially at right angles to the normal line of flight, so that the blades which in normal translation are the retracting blades sweep out areas which overlap, and a torsional balance pitch control system for obtaining cyclic pitch changes, whereby the blades of each rotor are interconnected to transmit pitch changing torques from one blade to another, each blade having the axis for pitch change nearer tothe leading edge than the centre of pressure axis and diverging from the centre of pressure axis towards the blade tip, whereby the lift of the retreating blades is reduced and subs'tantial increase in airflow through the double swept area is avoided.

2; A helicopter as claimed in claim 1, in which the pitch control system comprises a constant volume hydraulic system for each rotor responsive to aerodynamic forces for effecting cyclic pitch changes of the blades.

3. A helicopter as claimed in claim 1, in which all of the blades of each rotor are connected at their root ends to said pitch control system, said system including fluid pressure driven members connected to a-common fluid transmission circuit in such a manner that torques due to variations of lift of a blade are transmitted to the other blades in order to obtain cyclic pitch changes.

4. A helicopter, having two rotors arranged at a distance apart less than the rotor diameter and arranged to turn in opposite directions, each rotor having a plurality of blades each adapted to turn about a, longitudinal axis for pitch change, a constant volume hydraulic system for each rotor and fluid pressure driven members coupled with the rotor blades and connected to the hydraulic system of the corresponding rotor to transmit an impulse to. the hydraulic system when a blade rotates to decrease its pitch, each blade having its axis'of rotation for pitch change at an angle to the centre of pressure axis, the two axes diverging towards. the tip of the blade, and the axis for pitch change. being nearer to the leading edge of the blade than the centre of pressure axis, whereby in translation in a direction substantially at right angles to theline joiningthe centres of the rotor discs, with the retreating blades overlapping, substantial increase in airflow through the. double swept area is avoided.

JOHN SIDNEY NEALE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,870,928 Smith Aug. 9, 1932 2,344,967 Bennett Mar. 28, 1944 FOREIGN PATENTS Number Country Date 695,915 Germany Sept. 5, 1940 

